1. Field of the Invention
This invention relates generally to magnetic transducers for reading information signals from a magnetic medium and, in particular, to a soft antiparallel pinned spin valve sensor, and to magnetic recording systems which incorporate such sensors.
2. Description of Related Art
Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive read sensors, commonly referred to as MR heads, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an "MR element") as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization in the MR element and the direction of sense current flow through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material separated by a layer of non-magnetic material (e.g., copper) are generally referred to as spin valve (SV) sensors manifesting the GMR effect (also referred to as SV effect). In an SV sensor, one of the ferromagnetic layers, referred to as the pinned layer, has its magnetization typically pinned by exchange coupling with an antiferromagnetic (e.g., NiO or Fe--Mn) layer. The magnetization of the other ferromagnetic layer, referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the recorded magnetic medium (the signal field). In SV sensors, the SV effect varies as the cosine of the angle between the magnetization of the pinned layer and the magnetization of the free layer. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in direction of magnetization in the free layer, which in turn causes a change in resistance of the SV sensor and a corresponding change in the sensed current or voltage. IBM's U.S. Pat. No. 5,206,590 granted to Dieny et al. and incorporated herein by reference, discloses an MR sensor operating on the basis of the SV effect.
FIG. 1 shows a prior art SV sensor 100 comprising end regions 104 and 106 separated by a central region 102. A free layer (free ferromagnetic layer) 110 is separated from a pinned layer (pinned ferromagnetic layer) 120 by a non-magnetic, electrically-conducting spacer 115. The magnetization of the pinned layer 120 is fixed by an antiferromagnetic (AFM) layer 121. Free layer 110, spacer 115, pinned layer 120 and the AFM layer 121 are all formed in the central region 102. Hard bias layers 130 and 135 formed in the end regions 104 and 106, respectively, provide longitudinal bias for the free layer 110. Leads 140 and 145 formed over hard bias layers 130 and 135, respectively, provide electrical connections for the flow of the sensing current I.sub.s from a current source 160 to the MR sensor 100. Sensing means 170 connected to leads 140 and 145 sense the change in the resistance due to changes induced in the free layer 110 by the external magnetic field (e.g., field generated by a data bit stored on a disk).
Another type of spin valve sensors currently under development is an anti-parallel (AP)-pinned spin valve sensor. IBM's U.S. Pat. No. 5,583,725 granted to Coffey et al. and incorporated herein by reference, describes an AP-pinned SV sensor (FIG. 2) wherein the pinned layer is a laminated structure of two ferromagnetic layers separated by a non-magnetic coupling layer such that the magnetizations of the two ferromagnetic layers are strongly coupled together antiferromagnetically in an antiparallel orientation.
Referring to FIG. 2, there is shown a prior art AP-Pinned SV sensor 200 having a free layer 210 separated from a laminated AP-pinned layer 220 by a nonmagnetic, electrically-conducting spacer layer 215. Free layer 210 comprises a Co layer 212 and a Ni--Fe layer 214. The laminated AP-pinned layer 220 comprises a first ferromagnetic layer 222 and a second ferromagnetic layer 226 separated from each other by an antiparallel coupling (APC) layer 224 of nonmagnetic material. The two ferromagnetic layers 222 and 226 in the laminated AP-pinned layer 220 have their magnetization directions oriented antiparallel, as indicated by the head of the arrow 223 pointing out of the plane of the paper and the tail of the arrow 227 pointing into the plane of the paper. Antiferromagnetic (AFM) exchange biasing layers 230 and 232 formed on the lateral extensions 240 and 242 of the free layer 210. The AFM layers 230 and 232 longitudinally bias the free layer so its magnetization in the absence of an external field is in the direction of the arrow 250. Capping layers 260 and 262 provide corrosion resistance for the AFM layers 230 and 232, respectively. Electrical leads 270 and 272 provide electrical connections to current source 280 and a sensing means 285.
Coffey does not use a hard bias layer or an AFM layer adjacent to the pinned layer 220 for pinning the magnetization of the pinned ferromagnetic layer 220. Consequently, Coffey avoids the problems associated with the blocking temperature and/or corrosion of many AFM materials. However, according to Coffey once the sensor geometry is completed the directions of the magnetizations of first and second pinned layers 222 and 226 are set, perpendicular to the air bearing surface (plane of the disk), by applying a sufficiently large magnetic field (about 10 KOe). That is, a large external field is used in order to ensure that the spins are all pinned in the same direction. Once the spins are all pinned in the same direction, Coffey relies on the antiferromagnetic coupling between the two pinned layers 222 and 226 to maintain the pinning. FIGS. 2a and 2b are side views showing the spins directions in the pinned layers 222 and 226 before and after applying a large external field. Before applying a large external field, the spins are randomly formed in both pinned layers 222 and 226 (FIG. 2a). After applying a sufficiently large external field, the directions of the magnetizations in both pinned layers 222 and 226 become set meaning that the spins in each pinned layer become uniform in their directions (FIG. 2b).
However, there are two issues present in Coffey's AP-pinned SV sensor. First, if the magnetizations directions in the pinned layers 222 and 226 becomes disoriented due to a thermal asperity (actual or near contact between the head and the disk) or other unwanted fields (such as the field generated by the write head), there is no means for applying an external field of a large magnitude in the magnetic recording system to reset the directions of the magnetizations of the pinned layers and therefore, the SV sensor becomes inoperative, either partially or completely.
Second, even in the absence of a thermal asperity and/or an unwanted field, the magnetizations directions of the spins in the two pinned layers 222 and 226 rotate in the presence of an unwanted field because Coffey does not provide any means for keeping the directions of magnetizations of the pinned layers perpendicular to the air bearing surface. FIG. 2b shows the magnetizations directions of the spins perpendicular to the air bearing surface. FIGS. 2c and 2d show the directions of the magnetizations of the spins rotated in the presence of an unwanted (external) field such that the magnetizations directions is no longer perpendicular to the air bearing surface.
Rotation of the directions of the magnetizations in the pinned layers 222 and 226 result in read signal asymmetry, unpredictable read signal amplitude and free layer saturation.
Therefore there is a need for an AP-pinned SV sensor in which the directions of magnetizations in the pinned layers can be set by a rather small field, an AP-pinned SV sensor which does not use hard bias or AFM layers for pinning the pinned layers and an AP-pinned SV sensor in which the directions of magnetizations do not rotate in the presence of unwanted fields.